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Date:   Sun, 19 Mar 2023 10:15:55 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Takashi Sakamoto <o-takashi@...amocchi.jp>
Cc:     John Keeping <john@...anate.com>, Takashi Iwai <tiwai@...e.com>,
        "moderated list:SOUND" <alsa-devel@...a-project.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ALSA: usb-audio: Fix recursive locking on XRUN

On Sun, 19 Mar 2023 08:57:03 +0100,
Takashi Iwai wrote:
> 
> On Sun, 19 Mar 2023 04:28:53 +0100,
> Takashi Sakamoto wrote:
> > 
> > Hi,
> > 
> > On Sat, Mar 18, 2023 at 09:20:05AM +0900, Takashi Sakamoto wrote:
> > > On Fri, Mar 17, 2023 at 07:51:27PM +0000, John Keeping wrote:
> > > > snd_usb_queue_pending_output_urbs() may be called from
> > > > snd_pcm_ops::ack() which means the PCM stream is locked.
> > > > 
> > > > For the normal case where the call back into the PCM core is via
> > > > prepare_output_urb() the "_under_stream_lock" variant of
> > > > snd_pcm_period_elapsed() is called, but when an error occurs and the
> > > > stream is stopped as XRUN then snd_pcm_xrun() tries to recursively lock
> > > > the stream which results in deadlock.
> > > > 
> > > > Follow the example of snd_pcm_period_elapsed() by adding
> > > > snd_pcm_xrun_under_stream_lock() and use this when the PCM substream
> > > > lock is already held.
> > > > 
> > > > Signed-off-by: John Keeping <john@...anate.com>
> > > > ---
> > > >  include/sound/pcm.h     |  1 +
> > > >  sound/core/pcm_native.c | 28 ++++++++++++++++++++++++----
> > > >  sound/usb/endpoint.c    | 18 +++++++++++-------
> > > >  3 files changed, 36 insertions(+), 11 deletions(-)
> > >  
> > > The name of added kernel API implies me that you refer to existent
> > > 'snd_pcm_period_elapsed_under_stream_lock()' which I added to Linux
> > > v5.14.
> > > 
> > > In my opinion, unlike the version of period elapsed API, the version of
> > > XRUN API seems not to be necessarily required to ALSA PCM core, since PCM
> > > device drivers can implement .pointer callback in the part of PCM operation.
> > > When the callback returns SNDRV_PCM_POS_XRUN, ALSA PCM application get
> > > occurence of XRUN as a result of any operation relevant to hwptr movement
> > > (e.g. SNDRV_PCM_IOCTL_HWSYNC).
> > > 
> > > Therefore I think it possible to fix the issue without the proposed
> > > kernel API. I can assume some scenario:
> > > 
> > > 1. Failure at tasklet for URB completion
> > > 
> > > It is softIRQ context. The stream lock is not acquired. It doesn't
> > > matter to call current XRUN API.
> > > 
> > > 2. Failure at PCM operation called by ALSA PCM application
> > > 
> > > It is process context. The stream lock is acquired before calling driver
> > > code. When detecting any type of failure, driver code stores the state.
> > > Then .pointer callback should return SNDRV_PCM_POS_XRUNrefering to
> > > the state.
> > 
> > Although being inexperienced to hack driver for USB audio device class,
> > I attempt to post the patch to fix the issue of recursive stream lock.
> > I apologies in advance since the patch is not tested yet...
> > 
> > The 'in_xrun' member is newly added to 'struct snd_usb_substream'. When
> > detecting any failure, false is assigned to the member. The assignment
> > is expected to be done in both softIRQ context, and process context with
> > stream lock, thus no need to take care of cocurrent access (e.g. by usage
> > of WRITE_ONCE/READ_ONCE).
> > 
> > Typical ALSA PCM application periodically calls PCM operation which calls
> > .pointer in driver code. As I described, returning SNDRV_PCM_POS_XRUN
> > takes ALSA PCM core to handle XRUN state of PCM substream in the timing.
> > 
> > The negative point of the patch is the delay of XRUN notification to user
> > space application. In the point, I think the new kernel API introduced by
> > your patch has advantage.
> > 
> > The in_xrun member can be replaced with a kind of EP_STATE_
> > enumerations; i.e. EP_STATE_XRUN. In the case, we need some care so that
> > the state should be referred from pcm.c.
> 
> Thanks for the patch.  That would work, but the shortcoming side of
> this implementation is that it misses stopping / reporting the error
> immediately but waiting for the next pointer update.
> 
> It might be simpler if we perform the xrun handling in the caller
> side, i.e. a change like below:
> 
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -2155,6 +2155,8 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
>  		ret = substream->ops->ack(substream);
>  		if (ret < 0) {
>  			runtime->control->appl_ptr = old_appl_ptr;
> +			if (ret == -EPIPE)
> +				__snd_pcm_xrun(substream);
>  			return ret;
>  		}
>  	}
> 
> ... and let the caller returning -EPIPE for XRUN:

and that misses the XRUN in the case of non-stream-lock.
A revised version is below.


Takashi

-- 8< --
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -2155,6 +2155,8 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
 		ret = substream->ops->ack(substream);
 		if (ret < 0) {
 			runtime->control->appl_ptr = old_appl_ptr;
+			if (ret == -EPIPE)
+				__snd_pcm_xrun(substream);
 			return ret;
 		}
 	}
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -455,8 +455,8 @@ static void push_back_to_ready_list(struct snd_usb_endpoint *ep,
  * This function is used both for implicit feedback endpoints and in low-
  * latency playback mode.
  */
-void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
-				       bool in_stream_lock)
+int snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
+				      bool in_stream_lock)
 {
 	bool implicit_fb = snd_usb_endpoint_implicit_feedback_sink(ep);
 
@@ -480,7 +480,7 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
 		spin_unlock_irqrestore(&ep->lock, flags);
 
 		if (ctx == NULL)
-			return;
+			break;
 
 		/* copy over the length information */
 		if (implicit_fb) {
@@ -495,11 +495,14 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
 			break;
 		if (err < 0) {
 			/* push back to ready list again for -EAGAIN */
-			if (err == -EAGAIN)
+			if (err == -EAGAIN) {
 				push_back_to_ready_list(ep, ctx);
-			else
+				break;
+			}
+
+			if (!in_stream_lock)
 				notify_xrun(ep);
-			return;
+			return -EPIPE;
 		}
 
 		err = usb_submit_urb(ctx->urb, GFP_ATOMIC);
@@ -507,13 +510,16 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
 			usb_audio_err(ep->chip,
 				      "Unable to submit urb #%d: %d at %s\n",
 				      ctx->index, err, __func__);
-			notify_xrun(ep);
-			return;
+			if (!in_stream_lock)
+				notify_xrun(ep);
+			return -EPIPE;
 		}
 
 		set_bit(ctx->index, &ep->active_mask);
 		atomic_inc(&ep->submitted_urbs);
 	}
+
+	return 0;
 }
 
 /*
--- a/sound/usb/endpoint.h
+++ b/sound/usb/endpoint.h
@@ -52,7 +52,7 @@ int snd_usb_endpoint_implicit_feedback_sink(struct snd_usb_endpoint *ep);
 int snd_usb_endpoint_next_packet_size(struct snd_usb_endpoint *ep,
 				      struct snd_urb_ctx *ctx, int idx,
 				      unsigned int avail);
-void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
-				       bool in_stream_lock);
+int snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
+				      bool in_stream_lock);
 
 #endif /* __USBAUDIO_ENDPOINT_H */
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -1639,7 +1639,7 @@ static int snd_usb_pcm_playback_ack(struct snd_pcm_substream *substream)
 	 * outputs here
 	 */
 	if (!ep->active_mask)
-		snd_usb_queue_pending_output_urbs(ep, true);
+		return snd_usb_queue_pending_output_urbs(ep, true);
 	return 0;
 }
 

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